1. Field of the Invention
This invention pertains to telephone hold circuits, and in particular to single and double line hold circuits for connection across the tip and ring leads of a single telephone line or two telephone lines.
2. Description of the Prior Art
Single line hold circuits generally operate by providing a variable impedance means for connection between the tip and ring leads of a telephone line. Such means is normally in a high impedance state in which it does not affect normal telephone use. When it is desired to enter a hold mode, however, the impedance means is switched to a low impedance state in which it can maintain loop current through the line even if the telephone goes on-hook. Prior single line hold circuits have used both relays and semiconductor devices as the variable impedance means. Relay hold circuits suffer from the disadvantages that relays are bulky, expensive, and have comparatively limited operating lives. While prior single line hold circuits using semiconductors as the impedance means have not suffered from these disadvantages, they have been subject to even more basic drawbacks which have prevented them from achieving widespread acceptance in the telephone industry.
Prior semiconductor hold circuits have functioned by providing a thyristor, such as a silicon controlled rectifier, hereinafter referred to as an SCR, connected across the tip and ring leads of a telephone line. When untriggered, the SCR has a very high impedance. When it is desired to enter the hold mode, a signal is applied to the SCR gate, triggering it into a low impedance, conducting state in which it can maintain loop current despite the telephone going on-hook. The SCR will remain in its conducting state without further triggering for as long as loop current is supplied to it. However, when loop current is interrupted, even for very short intervals, the SCR immediately reverts to its high impedance state, thus taking the line out of the hold mode.
This instantaneous loss of the hold state due to short interruptions in loop current is a serious drawback present in all prior semiconductor hold circuits. To be of practical utility, a hold circuit must be able to maintain itself in a hold, i. e. low impedance, mode despite interruptions in loop current of up to several hundred milliseconds, but must automatically leave the hold mode for all interruptions of greater than 600 ms. The low limit is due to the fact that the central office or PBX to which telephone lines are connected frequently interrupt loop current for intervals of up to 300 ms. for a variety of reasons. The upper time limit corresponds to the loop current interruption which occurs when the central office or PBX switches a telephone line from an audio line to a howler circuit in response to the telephone line being off-hook or on hold for too great a period of time. When the switch to the howler circuit is made, the line must leave the hold mode, otherwise the line will be designated defective and disconnected.
Prior relay hold circuits have included means for maintaining the line in a hold mode despite interruptions in loop current of up to 300-600 ms. These circuits have operated by storing charge during hold conditions and pumping such charge through the relay coil during interruptions. There would be many practical difficulties in applying such a technique to semiconductor hold circuits, not the least of which would be that a single semiconductor device could not be utilized to match many telephone company input specifications which require that a hold circuit be suitable for use with a wide range of loop currents. The charge pumping technique also suffers from the drawback that a large amount of charge must be stored to span interruptions of 300-600 ms.
An additional requirement for a hold circuit of general utility is that the circuit have an input impedance of at least 10 megohms during non-hold conditions to satisfy FCC part 68 specifications for devices to be connected between the tip and ring terminals. Many prior single line hold circuits fail to meet this requirement.